The present invention relates to the field of quantum cascade lasers and, more particularly, to the measurement and characterization of their intersubband electroluminescence.
As has been described extensively in the prior art (see, for example, F. Capasso et al., IEEE J. Select. Topics in Quantum Electr., Vol.6 (6), p. 931-947 (2000)), a quantum cascade (QC) laser is based on intersubband transitions between excited states of coupled quantum wells, utilizing resonant tunneling as the pumping mechanism. Unlike all other semiconductor lasers (e.g., diode lasers), the wavelength of the laser emission of a QC laser is essentially determined by quantum confinement; i.e., by the thickness of the layers of the active region material. As such, it can be tailored over a very wide range without modifying the semiconductor material itself. For example, QC lasers with AlInAs/GaInAs active regions have operated at mid-infrared wavelengths in the 3 to 24 xcexcm range. In diode lasers, in contrast, the bandgap of the active region is the main factor in determining the lasing wavelength.
More specifically, diode lasers, including quantum well lasers, rely on transitions between energy bands in which conduction band electrons and valence band holes, injected into the active region through a forward-biased p-n junction, radiatively recombine across the bandgap. Thus, as noted above, the bandgap essentially determines the lasing wavelength. In contrast, the QC laser relies on only one type of carrier; i.e., it is a unipolar semiconductor laser in which electronic transitions between conduction band states arise from size quantization in the active region heterostructure.
Even though QC lasers have been the subject of much research and study since their initial development in 1994, the nature of their electron distribution function is still not well understood. An article by M. Troccoli et al., Applied Physics Letters, 77,1088-1090 (2000), discusses the possibility of extracting the excited state electronic population from the intersubband (ISB) emission of QC electroluminescence structures. Unfortunately, ISB electro-luminescence (EL), the best experimental probe for this type of analysis, in practice cannot be measured above laser threshold, due to the large intensity difference between laser emission and electroluminescence emission, as well as the lack of detectors with a large enough dynamic range.
The present invention relates to a technique for measuring intersubband electroluminescence (ISB-EL) above laser threshold which is accomplished by cleaving the laser stripe along its length and detecting the ISB spontaneous emission from the direction orthogonal to the axis of the optical cavity.
In particular, a QC laser is cleaved longitudinally (along the active region) and the laser facets are subsequently coated with a highly reflective material. A detector is disposed to capture the spontaneous emission which will exit from the active region along the cleaved face. The detector will also measure the scattered laser emission, resulting from defects in the laser material or waveguide, thereby allowing for the determination of the threshold current density (Jth) of the device.
In one embodiment of the present invention, the laser stripe is cleaved along the direction orthogonal to the laser facets and approximately in the middle of the waveguide, using a diamond tip device. The scattered nature of the light can be analyzed by its polarization; the ISB-EL emission is fully TM polarized, and laser light is unpolarized due to the scattering process.